A relatively new and rapidly growing market for polyolefin films is the modified atmosphere packaging (MAP) of fresh fruits, flowers and vegetables (hereinafter "fresh produce"). The modified atmosphere is generally desired to both maintain freshness and extend shelf life for longer periods than has been previously possible. Successful MAP design requires the proper balance among the following properties: clarity (for retail sale most importantly), excellent hot tack, seal strength to minimize package leaks, good organoleptics (minimum interaction by flavor influencing materials with the packaged product either as donor or acceptor (scalper)) and extractibility, as well as a versatile range of oxygen and carbon dioxide transmission rates, usually coupled with good moisture barrier.
Fresh produce offers new challenges for the packaging industry. Among these challenges are the "living" nature of these to-be-packaged items. The term "living" signifies that a reverse photosynthesis begins to occur at the time of picking or cutting. Such reverse photosynthesis includes the absorbing of oxygen and the giving off of CO.sub.2. MAP is based on knowledge of how a specific produce item's respiration rate responds to the atmosphere in which the produce resides. Proper choice of packaging materials can substantially extend the freshness, quality and shelf life. More extensive discussions of plant respiration may be found in "Postharvest: An Introduction to the Physiology and Handling of Fruits and Vegetables", AVI Publishing Co., Westport, Conn.
To successfully package various fresh produce, a container, film, or bag must have the ability to permit the passage of sufficient oxygen to substantially prevent the growth of anaerobic bacteria as such bacteria are among the causes of rotting or spoiling of fresh produce.
In the past, a film manufacturer has been limited in the ability to control or manipulate the permeability of a given film. Generally, the simplest, technique for obtaining increased permeability, has been to choose a lower density resin on which to base a film. However, selecting a lower density resin, while generally providing better clarity than higher density resins, has substantial practical problems, such a choice usually also leads to a softer film than higher density films. Such relative softness leads an additional problem known as reduced machineability. Conversely, by increasing density to improve machineabliity and freshness perception, clarity and gas permeability have traditionally been diminished.
Therefore, a commercial need exists for films with an improved physical property balance and increased gas transmission rate. The increasing of the gas transmission rate for a polyethylene of a given density would advantageously provide the means for producing a film with a relatively high gas transmission rate characteristic of lower densities, while maintaining stiffness, clarity and overall film strength characteristic of higher densities.